1. Field of the Invention
The present invention relates to an electroless gold plating solution for use in formation of a gold plate coating on an industrial electronic component such as a printed wiring board.
2. Description of the Related Art
A printed wiring board generally has a metal circuit pattern on and/or within the board, a metal such as copper, which has a low electrical resistance, is used for the circuit, and an exposed copper area is covered with nickel or a nickel alloy and further with gold. The nickel or the nickel alloy is used as a barrier metal for preventing oxidation and corrosion of the copper circuit and/or preventing migration of copper and gold and, moreover, the gold coating is formed in order to suppress oxidation of the nickel or the nickel alloy, maintain reliability of contacts, improve solderability, etc. When such a circuit is formed, plating with nickel or a nickel alloy is carried out after forming the copper pattern, and there is then further carried out gold electroplating, autocatalytic gold plating after displacement gold plating, or thick displacement gold plating after the nickel or nickel alloy. With regard to the autocatalytic gold plating, there has been a desire for a composition containing no hazardous cyanide compound, and in recent years cyanide-free autocatalytic electroless gold plating has started to be used.
Since in cyanide-free autocatalytic electroless gold plating there is no cyanide compound, which forms a stable complex with gold in aqueous solution, the solution cannot be kept stable, and the problem of gold particles forming in the solution or in the vicinity of the walls of a container containing the solution, that is, bath decomposition, easily occurs. The complex stability constants of, for example, a gold sulfite complex and a gold cyanide complex in aqueous solution are 10−10 and 10−38 respectively, and the gold cyanide complex is far more stable. Since the autocatalytic electroless gold plating is used in industrial electronic components such as printed wiring boards, it is preferable that the gold coverage capacity for a target area is stable. There is therefore a desire for cyanide-free autocatalytic electroless gold plating in which there is no bath decomposition or hardly any bath decomposition, and which can be used stably.
As a measure against the problem of bath decomposition occurring in a cyanide-free electroless gold plating solution containing, for example, sulfite—thiosulfate as a complexing agent, there is known a method 1) in which a compound that can form a complex with gold or with an impurity metal ion that is a main cause of decomposition is added so as to make the solution stable (JP, A, 3-294484), and a method 2) in which a compound that is adsorbed on the surface of gold to thereby suppress gold deposition due to autocatalytic action is added so as to make the solution stable (JP, A, 6-145996).
However, in the case of method 1), the deposition potential of gold changes, and the physical properties that would allow it to be used as a circuit cannot be obtained. In the case of method 2), the effect of suppressing gold deposition is too strong, excessive addition degrades the deposition characteristics in detailed areas, and if the effect is excessive gold is prevented from depositing at all, meaning that the concentration of the compound added has to be set precisely, which is a problem.
On the other hand, an example of electroless gold plating employing cytosine as a decomposition inhibitor has been reported by H. Honma et al., (Plating and Surface Finishing, Vol. 82, No. 4, 89–92 (1995)), in which it is reported that a bath can be stabilized by adding 0.1 to 100 mg/L of cytosine at a pH of 6.0.
However, in this publication only the effect of stabilizing a bath by cytosine is described, there is no clear indication of the influence on gold deposition in detailed areas of a printed wiring board, etc. or on the physical properties of the gold plate coating, and in the above-mentioned range the concentration of cytosine after heating could hardly be detected and the effect in stabilizing a gold plating solution was not sufficient. Even increasing the concentration of cytosine added could not give a sufficient stabilizing effect in practice.
As an example of a compound similar to cytosine being added to a plating solution, there is a case of a gold—tin alloy plating bath (JP, A, 2001-192886). This is for electroplating, and the compound is added mainly for the purpose of suppressing variation in the alloy composition and not for the purpose of suppressing bath decomposition.